Industrial slings used in rigging or to lift, load, tow and/or move heavy loads are well-known in the art. At one time, industrial slings were made exclusively of metal forming wire-rope or chains. Over the years, these industrial slings have become more flexible and stronger. However, despite these improvements, metal wire-rope slings still do not have the flexibility of non-metal (or synthetic) slings and have been largely replaced by non-metal slings.
Non-metal industrial slings may be made of natural or synthetic materials (especially those made of standard or high tenacity core yarns). Non-metal slings made of synthetic materials are usually called synthetic slings.
One advantage of synthetic slings is that they have a very high load-lifting performance (i.e., a high strength-to-weight ratio), which results in lighter, more flexible and even stronger slings than the heavier, relatively inflexible metal slings.
Synthetic slings typically have a load-bearing core inside an elongated, tubular cover. The core bears the entire weight of the load to be lifted while the cover's sole function is to protect the core from physical damage and environmental exposure.
The cover protects the entire length of the core from direct physical damage, such as sharp edges from the load and other objects that may come in contact with the sling. The cover also protects the core from molecular damage (e.g., chemicals/acids, ultraviolet degradation caused by sunlight, environmental pollutants, excessive heat under working conditions, etc.).
The load-bearing core of a synthetic sling is made of a number of core yarns (sometimes called core strands). Each core yarn is made of a plurality of threads. Some synthetic slings are made by winding the core yarns into an endless loop in which each run or loop of the core yarn is substantially parallel to every other loop (this may be referred to as load-bearing core having core yarns laid straight). The winding is usually performed on a machine having a motor-driven roller and a free-rolling roller set a specific distance away from the motor-driven roller. During the manufacturing process, the cover is “bunched” together in accordion-like fashion, and the core yarns are run straight through the cover. The distance between rollers is determined by the desired length of the sling to be made.
In manufacturing a sling, it is helpful to know how much weight the sling needs to support (i.e., the rated load) and how much force or weight each individual core yarn can support. From that data, the number of loops that are needed to make the load-bearing core for the rated load may be calculated. Many factors may impact these calculations, including the type of material selected for the core yarns, the diameter of the yarns, and the diameter of the threads used to make the core yarns.
One problem with current synthetic slings is that there are loops of core yarns of slightly different lengths. When the sling is placed under load, the force of the load is borne by the shortest loops of core yarn. In other words, a load-bearing core is designed to have “X” number of core yarns to be able to lift the rated load, but only a fraction of the “X” number of core yarns bear the weight of the load because of the differences in lengths of each loop. This configuration sometimes results in the shortest loops being damaged and eventually breaking because they are overloaded. When the shortest loops of core yarns break, the next shortest loops of core yarns support the load until they too are damaged and eventually break, and so on until the synthetic sling suffers a catastrophic failure.
It is known in the art of wire-rope slings to twist the metal wires together. Various methods exist to force the wire's strands to twist together which include using a spinning wheel with holes where the individual wire stands are fed through. However, it has been shown that a similar method does not work with synthetic yarns because the synthetic yarns do not have the rigidity of the wire strands. Previous to this invention, there was no way to twist the synthetic core yarns together without special machinery and substantial input of time.
Therefore, it is desirable to provide synthetic slings with core yarns of more uniform length. It is also desirable to provide synthetic slings that are less susceptible to damage and breakage.